The essence of National Cooperative Highway Research Program Project 24-29 is geotechnical site characterization in scour-relevant terms for use by hydraulic engineers. The project goal is to develop guidelines for evaluating scour at bridge foundations on rock that can be integrated with the procedures of Federal Highway Administration Hydraulic Engineering Circular No. 18 (HEC-18). Rock scour in natural open channels appears to be related to five processes: 1) physical and chemical weathering that prepares rock surfaces for subsequent scour, 2) dissolution of soluble rocks, 3) cavitation 4) erosion and abrasion of degradable rocks, and 5) quarrying and plucking of jointed hard rocks. The definition of 'rock' for scour purposes is just as problematic as the definition of rock for other engineering applications. The physical properties of rock material can range from strong soil to much stronger than the best concrete. Benchmark materials being considered for rock in the context of scour are concrete and mortar. Rock with characteristics less than that of mortar might be highly susceptible to scour when exposed to the normal range of stream flows during the life of a bridge. Quantifying the rate of rock scour is a challenge because it appears to be governed by a threshold hydraulic loading condition below which no scour occurs, but above which scour losses accumulate.

Five bridge sites representing a range of conditions were visited in 2008. Conditions evaluated consist of 1) over-consolidated ice-contact glacial till protected by a boulder armor, I-90 at Schoharie Creek, Montgomery County, New York, 2) Oligocene limestone, I-10 at Chipola River, Jackson County, Florida, 3) slaking Oligocene siltstone, SR-22 at Mill Creek, Polk County, Oregon, 4) stratified Jurassic sandstone and claystone forming a knickpoint and plunge pool, SR-262 at Montezuma Creek, San Juan County, Utah, and 5) Cretaceous siltstone, SR-273 at Sacramento River, Shasta County, California. A USGS stream gage on Schoharie Creek, New York, was also visited; this gage is in slabby Paleozoic sandstone.

Hydraulic loading conditions are being expressed in terms of stream power which can be accumulated over the life of a bridge structure. A probability-weighted approach has been developed for representing a statistically average year in terms of scour using data from Schoharie Creek in New York. NCHRP Project 24-29 activities include field, laboratory, and modeling studies to refine the approach to quantifying rock scour at bridge sites.

Jeffrey R. Keaton is a Senior Principal Engineering Geologist in the Los Angeles office of MACTEC Engineering and Consulting, Inc. He specializes in quantifying hazardous natural processes for use in design and risk analysis. He has written numerous articles regarding engineering geology mapping, applied geomorphology, debris flows, landslides, collapsible soils, subsidence, fault rupture, earthquake-induced liquefaction, earthquake ground motion, and case histories.

BS Geological Engineering from the University of Arizona (1971)
MS Engineering (Geotechnical) from the University of California, Los Angeles (1972)
PhD Geology from Texas A&M University (1988)

Professional Engineer in California, Utah, Alaska, and Arizona
Professional Geologist in California, Arizona, and Utah
Engineering Geologist in California and Washington.

Keaton is active in a number of professional societies. He is past president of AEG. Currently, he is chair of IAEG (International Association for Engineering Geology and the Environment) Commission No 1, Engineering Geological Characterisation and Visualisation, and a member of Commission No 19, 3D Terrestrial Laser Scanning Technology in the Geosciences.